The present invention relates to a method for measuring characteristic constants of an alternating current motor, such as a three phase induction motor, said constants being at least one of a primary and secondary combined leakage inductance (l.sub.1 +l.sub.2), resistances r.sub.1, r.sub.2, a combined resistance (r.sub.1 +r.sub.2) thereof, and a self-inductance L.sub.1, which constants are used as control constants for a speed sensorless vector control of the motor by using a voltage command value of an inverter apparatus for controlling the speed of the motor.
Generally, it has been required for the inverter or the like used for variable speed control of an induction motor to provide an improved high torque starting and speed control characteristic at a low speed. In order to meet this requirement, sensorless vector control has come into wide use in which speed control is carried out without using any speed sensor, nor any terminal voltage sensor of a motor, by controlling the induced voltage Em of an induction motor to keep it constant and by making the slip frequency proportional to the torque current thereof.
In order to maintain a constant induced voltage Em in such a control system, it is required to determine the primary voltage by compensating a voltage drop in an impedance on the primary side and to set up the motor constants, primary resistance r.sub.1 and leakage inductance (l.sub.1 +l.sub.2). In order to calculate a slip frequency command, a secondary resistance r.sub.2 is needed to be set up by reducing a measured value r.sub.1 from a measured value (r.sub.1 +r.sub.2).
Further, a generalized inverter may be required as a load to drive a motor, domestic- or foreign-made, the motor constants of which are not known. In this case, before normal operation, the motor constants are measured by using an inverter, and the constants are established as control constants, after which the motor is operated with sensorless vector control. Such a method of measuring the primary and secondary combined leakage inductance (l.sub.1 +l.sub.2) and the primary and secondary combined resistance (r.sub.1 +r.sub.2) is described, for example, in Japanese Patent Laid-open No. 60-183953 (1985).
Therein, an alternating current motor is energized by three-phase excitation under the stopped state of the motor (primary frequency= slip frequency ) by using a three-phase inverter, and then the values (l.sub.1 +l.sub.2) and (r.sub.1 +r.sub.2) are calculatingly measured from an output of an inverter output voltage detector and a detected value of the motor current.
Since this method needs an inverter output voltage sensor, the method is difficult to apply to a generalized inverter which does not have any voltage sensor. Further, when the inverter output voltage is increased under a low load operation, the motor begins to rotate due to the three-phase excitation, which creates a problem of preventing the measurement of the constants. In order to avoid this problem, there is a method where the motor constants are calculatingly measured from a detected inverter output voltage and a detected motor current when the motor is energized with single-phase excitation in order to prevent motor rotation, which method has been disclosed in the report, "An automatic measurement of motor constants for speed sensorless vector control: 1992 National Convention Record I.E.E. Japan, No. 619".
In this method, the fundamental wave components of voltage Va and Vb are obtained by means of the general Fourier transform, since the inverter output voltage is an alternating voltage formed of a pulse width modulation voltage. Similarly, the fundamental wave components of current Ia and Ib are obtained by means of Fourier transform. In this case, the detection errors in Va and Vb may depend on the sampling frequency of the input voltage, since the inverter output voltage is a pulse width modulation voltage. Therefore, the values Va, Vb, Ia and Ib are detected 256 times for each value and (l.sub.1 +l.sub.2) and (r.sub.1 +r.sub.2) are calculatingly measured based on each of these average values.
This method may be accurate, since the actual fundamental wave voltage of the pulse width modulation applied to the motor is detected. However, the method also needs an inverter output voltage sensor like the former method described above, which leads to high cost. Further, since the fundamental wave voltage and current are obtained by means of Fourier transform, the accuracy in detected values may vary depending on the sampling frequency. Since the sampling cycle should be short in order to improve accuracy, a comparatively high speed microprocessing unit may be required to perform the necessary alternating excitation operation and calculating operations for Va, Vb, Ia and Ib in every cycle. Furthermore, since each of the values Va, Vb, Ia and Ib is obtained through averaging 256 detected values, the measurement takes, for example, 0.02 second.times.256= approximately 5 seconds when the alternating current excitation is performed with a primary frequency f.sub.1 =50 Hz, which causes a disadvantage in that the measuring time is disadvantageously long.
A previous method of measuring a primary self-inductance L.sub.1 (=M+l.sub.1), which is one of various measuring constants of a motor, is calculatingly measured from a motor terminal voltage and a non-load current under a normal operating condition of the sole motor (non-load condition ) as described in the above referenced 1992 National Convention Record.
The conventional method of measuring a primary self-inductance L.sub.1 described above has disadvantages in that is not capable of carrying out the measuring under a load condition. Therefore, in a case where a generalized inverter is connected with various loads, there are disadvantages measuring the constant.
On the other hand, a method of measuring a primary self-inductance under a load condition is described in Japanese Patent Laid-open No. 61-92185 (1986). According to this method wherein a motor has a motor terminal voltage detector, the primary self-inductance is calculatingly measured from an output of the motor terminal voltage detector and a detected motor current under the condition of controlling the primary frequency such that, by detecting an induced voltage vector, the direction of the secondary inter-linkage magnetic flux of the vector may result in zero.
Although the conventional method described above is capable of measuring the primary self-inductance under a load condition, the method requires that the measurement take place under a condition where the direction of the secondary inter-linkage magnetic flux component of the induced voltage vector is determined. In order to detect the magnetic flux direction component, it is required to detect the motor terminal voltage in an alternating current state and to convert the three-phase alternating current into direct current for measurement. Therefore, there are disadvantages in that it is not possible to measure a primary self-inductance in a case of using a generalized inverter without voltage detectors in the output side of the inverter.